Typical metal pickling baths are operated on the basis of nitric, hydrofluoric, and/or hydrochloric acids. In addition to the economic viewpoints, the problems of these pickles lie in an undesirably high quantity of nitrate in the waste water to be treated. Pickles having replacement acids for nitric acid, such as sulfuric acid with a greatly reduced proportion of nitric acid, are known for reducing this nitrate load, but they are extremely disadvantageous in regard to the pickling quality and capacity.
Furthermore, recycling systems for separating free acids and salts, such as acid retardation and diffusion dialysis, are used in order to reduce the nitrate load in the waste water by reclaiming the free acids and therefore also make the disposal of the waste acids more cost-effective. The savings in acids achievable therewith are considerable, but do not really solve the actual nitrate problem, since large quantities of nitrate-containing waste water are still produced by the nitrate salts. If acid recycling systems are used, the predominant wastewater load due to nitrate no longer comes from the pickle baths, but rather from the attached rinsing baths and air washers, which are not recycled.
A suggestion for processing used pickling acid comes from the teaching of DE 38 25 857 A1, for example, according to which the used pickling acid, having a specific iron content and material ratio of fluoride/iron, is to be adjusted using alkaline material to pH 4 to 6 while forming a crystalline precipitate and the liquid phase is to be evaporated until dry, possibly after separating the precipitate.
Furthermore, a method for preparing metal-containing waste pickles which contain nitric acid and hydrofluoric acid comes from the disclosure of DE 39 06 791 A1, in which the waste pickle is introduced into a dialysis cell, which is delimited by selectively permeable membranes and positioned between an electrode pair whose anode and cathode spaces contain sulfur.
A thermal method, the roasting method, offers the most complete recycling of the pickling bath concentrate. In this case, the pickling acids are evaporated together with the water and the metals are roasted to form oxides. The acid residues of the metal salts are reclaimed as free acids in the distillate of the roaster. Therefore, the pickling bath concentrate may be treated almost without wastewater and waste.
In the article “Industrielle Oxidrohstoffe—Herstellung nach dem Andritz-Ruthner-Sprührostverfahren [Industrial Oxide Raw Materials—Production According to the Andritz-Ruthner Spray Roasting Method]” by Dr. Wolfgang Kladnig, Sprechsaal, Vol. 124, Number 11/12, 1991, a method for the industrial production of oxide raw materials is described, in which a metal chloride solution is first produced by adding hydrochloric acid. The metal salt solution thus prepared is subsequently purified and subjected to pyrohydrolysis, in which the metal oxides to be extracted and hydrogen chloride gas form. While the metal oxides are subjected to still further purification steps, the hydrochloride gas is converted back into hydrochloric acid by using water. The hydrochloric acid thus obtained is used again for renewed production of a metal chloride solution.
A method for recycling metal pickling baths, in which the undesired compounds from the metal pickling baths, such as silicon, aluminum, and chromium compounds, are to be removed, is known from EP-A-O 578 537. For this purpose, scrap metal is added to the metal pickling baths to be purified in a first step, in order to neutralize the free acidity of the metal pickling baths, the neutralization having to be performed under a neutral gas atmosphere in order to prevent undesired secondary reactions of iron compounds contained in the metal pickling baths to form trivalent iron compounds. Subsequently, the solids contained in the neutralized metal pickling bath are filtered out. Because the acidity of the metal pickling bath is intentionally reduced, i.e., the pH value is increased, undesired compounds, such as silicon, aluminum, and chromium compounds, which dissolve at lower pH values, precipitate, through which the metal pickling bath may be purified of these compounds. Subsequently, the purified metal pickling bath is supplied back to the pickling process.
Thus, according to the related art according to EP 0 296 147 A1, a method for obtaining and/or reclaiming acids from metal-containing solutions of these acids is described, according to which the solutions are spray-roasted in a reactor at temperatures from 200 to 500° C. and subjected to subsequent absorption and condensation of the resulting gasses in columns at temperatures from 0 to 70° C.
However, the roasting method is energy-consuming, the energy consumption being directly proportional to the supply volume and approximately 100 m3 natural gas being consumed per 1 m3 supply volume. Since the roasting method evaporates water and acids to the same extent, the rinsing and exhaust air wastewater, which are diluted, may not be roasted directly. Because of the high proportion of water, the acid concentrations would be too small and/or the volume would be too large to return them to the pickling bath. The rinsing water must therefore still be treated in a wastewater system. Since the material load of this wastewater, above all the nitrates, may easily be 50% of the total nitric acid consumption, the roasting method per se, as it has been used until now, is not the comprehensive solution, especially in regard to the nitrate load of the wastewater.
The goal must therefore be to concentrate the strongly diluted wastewater from the rinsing and exhaust air washers enough that it may be introduced into the roast process. The concentration of the diluted wastewater has not been able to be implemented until now, however, since the technologies available are not usable. Thus, membrane technologies in the form of electrodialysis and reverse osmosis facilities could not be used because of the inadequate membrane strengths. Evaporation facilities are not usable because of the vapor volatility of nitric acid and hydrofluoric acid in the distillate. If free hydrofluoric and nitric acid are present in the supply to the evaporator, up to 50% of these free acids are found again in the distillate, so that usage of the distillate as rinsing water is not possible. The distillate, which now thus only contain 50% of the original nitrate load, must nonetheless be disposed of via the wastewater facility and the evaporator, up to 50% of these free acids are found again in the distillate, so that usage of the distillate as rinsing water is not possible. The distillate, which now therefore contains only 50% of the original nitrate load, must nonetheless be disposed of via the wastewater facility and would therefore again not comprehensively solve the nitrate problem in the wastewater.